【Weekly Topic】Things to Note When Installing Pipes (7.17-7.24)

1 Pipeline installation shall meet the following conditions: (1) The civil engineering works related to the pipelines have been inspected and found satisfactory, meeting the installation requirements； (2) The equipment connected to the pipeline has been properly aligned and secured ； (3) Relevant procedures that must be completed before pipe installation, such as cleaning, degreasing, and internal acid washing, have been carried out ； (4) Pipes, pipe fittings, pipeline accessories, valves, etc. have been inspected and found satisfactory, and are accompanied by relevant technical certificates ； (5) Tubes, pipe fittings, valves, etc. have been checked and confirmed to meet the design requirements; their interiors have been cleaned thoroughly, with no debris present. 2 When prefabricated components are used for pipe installation, these components must possess sufficient rigidity; they should not undergo any permanent deformation after lifting, and their temporary fixation must be secure and reliable. 3 Before the pipes are combined or the assembly is installed, the interior of the pipes must be cleaned thoroughly; no debris should remain inside them, and temporary seals should be installed. 4 The direction and gradient of the horizontal sections of the pipes shall meet the design requirements. When there are no specific requirements in the design, the determination of the pipe slope direction should be based on the principle of facilitating drainage, water release, and air expulsion. Its slope shall meet the requirements of DLGJ23 \"Technical Specifications for the Design of Steam and Water Pipelines in Thermal Power Plants\". When installing a Type II compensator in a horizontal position on a pipe that has a slope, the pipe sections on either side of the compensator should remain horizontal, while the middle section should be aligned with the slope direction of the pipe. 5 The position of the pipe butt weld shall comply with the design specifications. Otherwise, the following requirements must be met: (1) The position of the weld shall be at a distance from the starting point of the bend that is not less than the outer diameter of the pipe or 100 mm; (2) The distance between the two butt welds on the pipe shall be not less than the outer diameter of the pipe, and also not less than 150 mm ； (3) The position of the pipe portion of the support or hanger must not overlap with the butt weld of the pipe; the distance between the weld and the edge of the support or hanger must be at least 50 mm. For joints that require heat treatment after welding, this distance must be at least 5 times the width of the weld, and in any case not less than 100 mm ； (4) The pipe joints should be positioned away from openings for drain pipes, blowdown pipes, instrument pipes, etc. The distance from the edge of such openings should be no less than 50 mm, and also no less than the diameter of the opening ； (5) When pipes pass through partition walls or floor slabs, there shall be no joints in the sections of the pipes that are located within those partition walls or floor slabs. 6 When the two forming parts on the pipeline are welded to each other, short pipes should be added as per the design. 7 Unless the design requires cold drawing or thermal tightening, when connecting pipes, methods such as forced alignment, heating the pipes, using additional shims, or multiple layers of shims shall not be employed to eliminate defects such as gaps, misalignment, misfit, or out-of-centerness at the joint surfaces. The connection between pipes and equipment should occur naturally after the equipment has been installed and its anchor bolts tightened. 8 The groove type and dimensions of the pipe shall be determined according to the design drawings. When no specific requirements are given in the design, processing shall be carried out in accordance with the provisions of DL5007 Technical Specifications for Construction and Acceptance of Power Generation Facilities (Welding in Thermal Power Plants). 9 The requirements for the alignment quality of pipes or pipe fittings shall comply with the provisions of DL5007 \"Technical Specifications for Construction and Acceptance of Electric Power Projects (Welding Section for Thermal Power Plants)\". The paint, scale, rust, and other contaminants on the grooves of the 10 pipes and fittings, as well as on their inner and outer surfaces within a range of 10–15 mm, must be removed completely before alignment, until the metallic luster is visible. For grooves with a wall thickness of 20 mm or more, it is necessary to check for defects such as cracks and delaminations. 11. When aligning pipes, they should generally be straight. The angular deformation after welding is measured at a distance of 200 mm from the center of the joint. Unless otherwise specified, the permissible deviation ‘a’ for such bending shall be as follows: when the nominal pipe diameter DN < 100 mm, a ≤ 2 mm; when the nominal pipe diameter DN ≥ 100 mm, a ≤ 3 mm. 12. After the pipes have been properly aligned, they must be securely braced to prevent any movement during welding or heat treatment. 13 Cold drawing of pipes must comply with design specifications. The following requirements must be met prior to cold drawing: (1) All fixing supports in the cold drawing area must be securely installed; all welds between these supports (except for the cold drawing joints) must be completed and inspected as satisfactory; welds requiring heat treatment must have undergone such treatment ； (2) All supports and hangers have been installed; sufficient adjustment margin should be reserved for the hanger rods near the cold-drawn joints. The springs of the spring hangers should be pre-compressed to the design values and temporarily fixed, so that the springs do not bear any loads beyond the specified values ； (3) The direction and gradient of the pipeline shall meet the design requirements ； (4) The connection bolts between the flange and the valve have been tightened. After cold drawing of the pipe, the welds must pass inspection. Weld joints that require heat treatment must undergo overheating treatment before the tensioning devices can be removed. 14 The waveform compensator shall be stretched or compressed as specified in the design. Releasing the tensioning device should be done after pipeline installation is completed. When equipped with a sleeve inside, it should be installed correctly according to the flow direction of the medium (the fixed end of the sleeve is on the inlet side of the medium). The compensator connected to the equipment should be attached only after the equipment is finally fixed in place. 15 When installing flow orifice plates (or nozzles), the technical requirements for piping shall comply with the provisions of SDJ279 “Technical Specifications for Construction and Acceptance of Electric Power Projects (Part on Thermal Instruments and Control Devices)”. 16 If the pipeline installation work is interrupted, the pipe ends should be sealed promptly. 17 The permissible deviation values for pipe installation shall comply with the provisions in Table 17. Table 17 Allowable deviation values for pipe installation Item Allowable deviation (mm) Indoor: <±10 Above ground, outdoor: <±15 Indoor: <±15 In trenches, outdoor: <±15 Elevation: For buried pipes, <±20; for DN≤100, 1/1000 and ≤20 Curvature of horizontal pipes: For DN>100, 1.5/1000 and ≤20 Verticality of vertical pipes: ≤2/1000 and ≤15 Deviation in spacing between intersecting pipes: <±10 Note: DN refers to the nominal diameter of the pipe. The installation of 18 hangers should be carried out simultaneously with the pipeline installation. 19 On pipelines, when holes need to be made for installing instrument sockets, drain seats, etc., and the diameter of such holes is less than 30 mm, gas cutting shall not be used to create these holes.

(1) The general requirements for pipeline layout design are； 1) The pipeline layout design shall meet the requirements of the process piping and instrumentation diagram ； 2) The piping layout should be planned comprehensively to ensure safety, reliability, economic efficiency, and compliance with requirements related to construction, operation, and maintenance; it should also strive to be neat and aesthetically pleasing ； 3) When determining the orientation and laying method of the pipes for the inlet and outlet devices (units), coordination between the interior and exterior aspects should be ensured ； 4) The installation of plant-wide pipelines within the factory area should be carried out in coordination with the equipment (units), roads, and buildings in that area. Coordinate with structures, etc., to prevent pipelines from encircling installations (units), and to minimize intersections between pipelines and railways/roads ； 5) Pipes should be installed overhead or above ground ； If necessary, it can be buried or laid in a trench ； 6) Piping should be arranged in rows. Pipes on the ground should be laid on pipe racks or pipe supports ； 7) When arranging pipes on pipe racks or pipe supports, it is advisable to ensure that the vertical and horizontal loads acting on the pipe racks or pipe supports are balanced ； 8) A 10%–30% margin should be reserved on plant-wide pipe racks or pipe supports (including those passing through culverts), taking their loads into account. A 10%–20% margin should be reserved for the main corridor pipe supports of the installation, taking their loads into consideration ； 9) The layout of pipelines with special requirements regarding distance, angle, elevation differences, etc., as well as large-diameter pipelines, shall comply with the requirements of the equipment layout design ； 10) The piping layout shall not hinder the installation, maintenance of equipment, pumps and their internal components, nor the passage of fire trucks ； 11) The piping layout should provide the piping system with the necessary flexibility. Provided that the flexibility of the piping and the forces and moments exerted by the piping on equipment and pump nozzles remain within permissible limits, the piping should be made as short as possible with the fewest possible components ； 12) The placement of support points should be considered concurrently with pipeline planning. It is advisable to utilize the natural shape of the pipeline to achieve self-compensation ； 13) The pipeline layout should be arranged in a manner that gradually increases or decreases in level, in order to reduce the formation of air pockets or liquid pockets. When it is inevitable, venting and draining should be provided in accordance with operational and maintenance requirements. The piping layout should minimize \"blind loops\"” ； 14) When a pipeline for gas-liquid two-phase flow is divided into two or more branches, the piping layout should take symmetry into account or meet the requirements of the piping and instrumentation diagram. (2) Pipes and valves. Except for instruments, equipment, etc. that require flange or threaded connections, welding connections should be used. Flanges, threads, or other detachable connections should be considered in the following situations: 1) When disassembly is necessary for maintenance, cleaning, or purging ； 2) Lined pipe or jacketed pipe ； 3) Piping composed of two sections of different materials that are not suitable for welding connection ； 4) Pipe joints where on-site heat treatment of welds is difficult ； 5) Galvanized pipes with a nominal diameter of 100 mm or less ； 6) Determine the location for installing the blind plate or the \"8\"-shaped blind plate. (3) The gas branch pipe should be connected from the top of the main pipe. (4) Pipelines for toxic media shall be connected by welding; flanged or threaded connections shall not be used except where there are special requirements. Pipes carrying toxic media should be clearly marked to distinguish them from other pipes, and such pipes must not be laid underground. (5) When installing pipes for solid materials or pipes containing solid materials, the pipes should be made as short as possible. Fewer bends and no dead corners: 1) The connection between the branch pipe for solid materials and the main pipe should be made at an angle along the flow direction of the medium, with the angle not exceeding 45° ； 2) The bending radius of the elbow on the pipeline for solid materials should not be less than 6 times the nominal diameter of the pipeline ； 3) Piping for slurries containing large amounts of solid material and piping for high-viscosity liquids should have a slope. (6) For pipes that require thermal compensation, the entire piping system should be analyzed from its starting point to its ending point in order to determine a suitable thermal compensation scheme. (7) For pipes laid on pipe trays that require a slope, the height of the pipe supports can be adjusted. This can be achieved by adding steel sections or steel plate shims to the pipe supports. The vent gas main (or flare header) should be installed at the top of the pipe rack columns to facilitate elevation adjustments. (8) When installing the pipes connected to rotating mechanical equipment, the piping system should have sufficient flexibility to meet the allowable stress requirements at the equipment’s pipe connections. If necessary, the following measures can be taken: 1) Change the pipeline route to enhance natural compensation capacity ； 2) Select spring hangers and supports ； 3) Use metal bellows compensators ； 4) Install limit brackets in appropriate positions. (9) When laying out pipes connected to reciprocating compressors, the natural frequency of the mechanical vibrations of the piping system and the natural frequency of the air column within the pipes should be kept away from the excitation frequency of the machine. If necessary, the following measures can be taken: 1) Install additional vibration isolation supports ； 2) Appropriately increase the pipe diameter ； 3) Add a pulsation dampener or orifice plate ； 4) Set buffers reasonably, avoid resonant tube lengths, and minimize elbows as much as possible. (10) Branch pipes should not be installed at locations on the vibrating pipeline where bending moments are high. (11) At the bends in pipes prone to vibration (such as the outlet pipes of reciprocating compressors and reciprocating pumps), elbows with a bending radius of not less than 1.5 times the nominal diameter should be used. The branch pipe allows the medium to flow smoothly outward to the external connection. (12) When a branch pipe with a nominal diameter of 40 mm or less is connected to a pipe where vibration may occur, reinforcement measures shall be taken at the connection point, regardless of whether there is a valve on the branch pipe. (13) Horizontal pipes with self-flow should have a slope of not less than 3‰ in the direction of fluid flow. (14) When pipes pass through the floor, roof, or walls of a building, sleeves should be used, and any gaps between the sleeves and the pipe ends must be sealed. The diameter of the sleeve should be greater than the outer diameter of the pipeline insulation layer. It shall not affect the thermal displacement of the pipeline. Welds on the pipeline shall not be inside the casing, and shall be at least 150 mm away from the end of the casing. The sleeve should be 50 mm above the floor slab and roof surface. A rain cover should be installed when the pipe passes through the roof. Pipes should not pass through firewalls or explosion-proof walls. (15) When installing corrosive media, toxic media, and high-pressure pipelines, it is necessary to prevent harm to personnel and equipment resulting from leaks at flanges, threads, gasket seals, etc. Leak-prone areas should be avoided above walkways or pumps; otherwise, safety protections should be installed. (16) For pipes with insulation, pipe supports should be installed at pipe piers and pipe racks. For pipes without insulation, pipe supports may not be installed if not required. When the thickness of the insulation layer is 80 mm or less, a pipe support 100 mm in height should be used ； When the thickness of the insulation layer is greater than 80 mm, use pipe supports that are 150 mm in height ； When the thickness of the insulation layer is greater than 130 mm, use pipe supports that are 200 mm high ； Insulated pipe supports should be used for insulated pipes. (l7) When there are significant elevation differences in the plant area, the installation of pipelines throughout the plant should be adjusted to match these elevation differences. Adjust the elevation of the pipe gallery at the appropriate location. The minimum slope of the pipeline should be 2‰. The pipe slope change point should be located at a turn or near a fixed point. (18) For pipelines that cross or pass over railways and roads within the plant area, no pipeline components such as valves, metal bellows compensators, flanges, or threaded joints shall be installed at the crossing or passing sections. (19) For buried pipes with thermal displacement, retaining piles can be installed provided the pipe’s curvature permits it; otherwise, thermal compensation measures should be adopted. (20) When arranging pipes, the placement of pipe welds shall meet the following requirements: 1) The distance between the center of the pipe’s butt weld and the starting point of the bend shall not be less than the outer diameter of the pipe; it shall also be no less than 100 mm ； 2) Center distance between two adjacent butt welds on the pipe: a. For pipes with a nominal diameter of less than 150 mm, it shall not be less than the outer diameter, nor less than 50 mm ； b. For pipes with a nominal diameter of 150 mm or greater, it shall not be less than 150 mm ； 3) The clear distance between the circumferential weld and the edge of the support or hanger should not be less than 50 mm ； The minimum clear distance between a weld that requires heat treatment and the edge of a support or hanger should be greater than 5 times the width of the weld, and must not be less than 100 mm.

Reply to 1# zgj2405: 1. Install in accordance with the drawings for pipes, valves, and instruments. The electric pump should be fixed in an appropriate location, a proper foundation should be constructed, and it should be tested after being connected to the power supply. 2. The safety valve should be installed after the hydrostatic test. The safety valve should be connected to a steam discharge pipe to direct the steam to a safe location in the boiler room. The safety valve should be installed vertically. Valves are not allowed to be installed on the guide tube. 3. The waste discharge pipe should be connected to a waste container or some other safe location, and the pipe must be secured to prevent accidents such as burns that may occur during waste discharge. 4. The steam pipes other than those connected to the main steam valve are installed by the user unit. There must be at least one elbow on the pipeline connected to the main steam valve; if not, an expansion joint should be added, and the outer wall of the pipeline should be insulated. 5. The drain pipe at the bottom of the water level gauge must be connected to a safe location. The valve of the water level gauge should rotate smoothly without leaking water. Lighting equipment must be installed at the water level gauge area, and \"red lines\" should be marked to indicate the highest, normal, and lowest safe water levels, to facilitate observation. Easy to observe. 6. After the pressure gauge is installed, a red line should be marked on its dial to indicate the maximum allowable operating pressure of the boiler. The valve attached to the pressure gauge must operate smoothly and must not leak; the drain system should be unobstructed. Insulation is not allowed on the pressure gauge tubing.

Reply to 1# zgj2405: A pipeline is a device made up of pipes, pipe fittings, valves, etc., used for transporting gases, liquids, or fluids containing solid particles. Typically, after being pressurized by blowers, compressors, pumps, boilers, etc., fluids flow from areas of high pressure to areas of low pressure in pipelines; they can also be transported using their own pressure or gravity. Pipes have a wide range of uses, mainly in water supply, drainage, heating, gas supply, long-distance transportation of oil and natural gas, agricultural irrigation, hydraulic engineering, and various industrial installations. When the flow rate of the fluid is known, the diameter of the pipe depends on the allowable flow velocity or the allowable frictional loss (pressure drop). When the flow rate is high, the pipe diameter is small, but the pressure drop increases. Therefore, high flow rates can save on pipeline infrastructure costs, but the operating energy expenses for power equipment such as pumps and compressors increase. Furthermore, if the flow rate is too high, it may also lead to some other adverse effects. Therefore, the pipe diameter should be determined through comprehensive consideration of construction costs, operating expenses, and other technical factors. The connection methods between pipes, pipe fittings, valves, and the inlet and outlet connections on equipment are determined by factors such as the properties of the fluid, pressure and temperature, as well as the material, size of the pipes and their installation location. The main methods include threaded connection, flanged connection, socket connection, and welding. Threaded connections are mainly suitable for small-diameter pipes. When connecting, it is generally necessary to wrap fluoroplastic sealing tape around the threaded connection area, or apply thick paint or wrap it with materials such as hemp fiber as sealing agents, in order to prevent leaks. At pressures above 1.6 MPa, gaskets are generally used for sealing at the pipe ends. This connection method is simple and allows for disassembly and reinstallation; however, union fittings must be installed at appropriate locations on the pipes to facilitate this process. Flange connections are suitable for a wide range of pipe diameters. When connecting, different flanges and gaskets are selected based on the properties, pressure, and temperature of the fluid, with bolts used to clamp the gaskets in order to maintain sealing. Flange connections are commonly used at pipe sections that require frequent disassembly and assembly, as well as where pipes are connected to equipment. Plug-and-socket connections are mainly used for connecting cast iron pipes, concrete pipes, clay pipes, and their fittings, and are suitable only for water supply, drainage, and gas pipelines operating under low pressure and normal temperature conditions. During connection, hemp fibers, cotton thread, or asbestos rope are generally inserted first into the grooves of the socket and spigot, after which materials such as asbestos cement or lead are used to fill those grooves. A rubber sealing ring can also be placed inside the socket and spigot to provide better flexibility and allow for slight movement of the pipes. Welded joints offer the best strength and sealing performance; they are suitable for various types of pipes, saving labor and materials. However, the pipes and their connectors must be cut in order to disassemble them. In cities, the main pipelines for water supply, drainage, heating, and gas supply, as well as long-distance oil and gas pipelines, are mostly laid underground, whereas the process pipelines in factories are usually installed above ground to facilitate operation and maintenance. Issues such as pipeline passage, support, slope and drainage/ventilation, compensation, insulation and heating, corrosion protection and cleaning, identification and painting, as well as safety, are all important matters whether the pipelines are installed above ground or underground. Pipelines on the ground should avoid crossing roads, railways, and waterways as much as possible. When crossing cannot be avoided, the height of the crossing should also allow pedestrians and vehicles to pass safely. Underground pipes are generally laid along roads, with appropriate distances maintained between different types of pipes to facilitate installation and maintenance ； The surface of the heating pipeline is covered with an insulating layer, and it is laid in trenches or protective pipes to prevent it from being damaged by soil pressure and to allow the pipe to expand and move. Pipelines may be subjected to a variety of external forces, including their own weight, the thrust exerted by fluids at the pipe ends, wind and snow loads, soil pressure, thermal stresses caused by heat expansion and contraction, vibration loads, and **disasters. To ensure the strength and stiffness of the pipeline, various supports (hangers) must be installed, such as movable supports, fixed supports, guide supports, and spring supports. The placement of the support is determined by factors such as the pipe diameter, material, wall thickness, and load. Fixed supports are used to control the thermal expansion of pipes in segments, ensuring uniform operation of the expansion joints ； Guiding supports allow the pipe to move only axially; in order to remove condensed water, steam, and other water-containing gases, pipes should have a certain slope, generally not less than 2 per thousand. For underground drainage pipes that utilize gravity flow, the slope should be no less than 0.5%. Steam or other water-containing gas pipelines are equipped with drain pipes or steam traps at their lowest points; some of these pipelines also have gas-water separators to allow water to be removed promptly, thereby preventing water hammer in the pipes and hindering gas flow. Water supply or other liquid pipelines are equipped with exhaust devices at their highest points to remove air or other gases accumulated within the pipes, thereby preventing operational abnormalities caused by air blockages. If pipes cannot expand and contract freely, huge additional stresses are generated. Therefore, in pipes with large temperature variations and in normal-temperature pipes that require free movement, expansion joints must be installed to compensate for the expansion and contraction of the pipes and eliminate the effects of additional stresses. For steam pipes, high-temperature pipes, low-temperature pipes, as well as those that require protection against heat and freezing, insulation materials need to be used to cover the outside of the pipes in order to prevent loss of heat (or cold) from inside the pipes or to avoid freezing. For certain liquid pipelines with high freezing points, heating and insulation are also required to prevent the liquid from becoming too viscous or freezing, which could affect transportation. Common insulation materials include cement perlite, glass wool, rock wool, and asbestos diatomite. To prevent soil erosion, the surfaces of underground metal pipes should be coated with anti-rust paint or corrosion-resistant coatings such as tar and asphalt, or covered with glass cloth and burlap impregnated with asphalt. Pipelines buried in highly corrosive, low-resistance soil must be equipped with cathodic protection to prevent corrosion. To prevent atmospheric corrosion, steel pipes on the ground are usually coated with various anti-rust paints on their surfaces. All types of pipes should be cleaned thoroughly before use, and some pipes also require regular cleaning of their interiors. For easy cleaning, filters or purge and cleaning ports are installed on the pipes. On pipelines for long-distance transportation of oil and natural gas, cleaners must be used regularly to remove debris accumulated inside the pipes; for this purpose, specialized devices for sending and receiving cleaners are required. When there are many types of pipes, to facilitate operation and maintenance, they are painted with specified colors on their surfaces for identification. For example, steam pipes are colored red, compressed air pipes are light blue, etc. To ensure the safe operation of pipelines and to prevent the escalation of accidents in case they occur, in addition to installing monitoring and control instruments as well as safety valves on the pipelines, special safety measures are taken for certain important pipelines. For example, accident relief valves or emergency shut-off valves are installed on gas pipelines and pipelines used for transporting oil and natural gas over long distances. They can automatically and promptly halt transportation in the event of a catastrophic accident, thereby minimizing disaster losses. Metallic materials for pipelines – I. Selection of metallic materials for pipelines 1. Characteristics of metallic materials used in pressure pipelines Pressure pipelines are utilized in various industries. The basic requirements for them are “safety and usability”. Safety is essential for proper usage; usage must also be safe. Additionally, economic considerations come into play—namely, low investment costs and a long service life. Naturally, this depends on many factors. Materials are the foundation of engineering; first and foremost, we must understand the special requirements for metallic materials used in pressure pipelines. In addition to bearing loads, pressure pipes are also subjected to special stresses due to operating in different environments, temperatures, and media. (1) Changes in the properties of metallic materials at high temperatures ① Creep When steel is subjected to external forces at high temperatures, a phenomenon in which it undergoes slow and continuous plastic deformation over time is known as creep. The creep characteristics of steel are closely related to temperature and stress. As temperature rises or stress increases, the creep rate accelerates. For example, creep occurs when the operating temperature exceeds 300–350°C for carbon steel, and 300–400°C for alloy steel. The stress required to induce creep is lower than the yield strength of the steel at the test temperature. Therefore, the steel used for boilers, steam pipes, and pressure vessels that operate at high temperatures for extended periods must possess good creep resistance, so as to prevent excessive deformation caused by creep, which could lead to structural failure and serious accidents such as explosions. ② Spheroidization and graphitization: Under the effect of high temperatures, the cementite in carbon steel gains energy, causing it to migrate and aggregate. This results in the formation of coarse-grained cementite particles dispersed within ferrite. Gradually, these cementite particles transform from lamellar shapes into spherical ones; this process is known as spheroidization. Due to the extremely low strength of graphite and its flaky structure, the strength of the material is **reduced** and its brittleness increases; this phenomenon is known as the graphitization of the material. When carbon steel is exposed to environments above 425°C for an extended period, graphitization occurs, and this phenomenon becomes more pronounced at temperatures above 475°C. SH3059 specifies that the maximum operating temperature for carbon steel is 425°C, while GB150 specifies it as 450°C. ③ Thermal fatigue performance: If steel is subjected to repeated cycles of heating and cooling over a long period of time, thermal stresses caused by these temperature changes can lead to the formation of tiny cracks within the material, which then continue to expand until the material eventually breaks. Therefore, in structural and piping systems operating under conditions of temperature fluctuations, the thermal fatigue properties of the steel should be taken into account. ④ High-temperature oxidation of materials: Metal materials will oxidize in an environment with high-temperature oxidative media (such as flue gases), forming an oxide scale that is prone to cracking and falling off. Carbon steel is prone to forming scale in high-temperature gases at 570°C, which causes the metal to thin out. Therefore, steel pipes for gas, flue gas, etc., should be limited to operating at 560°C. (2) Changes in the properties of metal materials at low temperatures: When the ambient temperature falls below the material’s critical temperature, its impact toughness decreases sharply; this critical temperature is known as the material’s brittle transition temperature. The low-temperature impact toughness (impact energy) is commonly used to measure a material’s toughness at low temperatures. For pipes that operate in low-temperature conditions, it is essential to pay attention to their low-temperature impact toughness. (3) Changes in the performance of pipes in corrosive environments. The media flowing through pipes in industries such as petrochemicals, shipping, and offshore oil platforms are often corrosive. It has been proven that the hazards caused by metal corrosion are widespread and severe; corrosion can result in direct or indirect losses. For example, stress corrosion, fatigue corrosion, and intergranular corrosion in metals often lead to catastrophic accidents, while metal corrosion results in significant consumption of metal and a waste of numerous resources. The main media that cause corrosion are as follows. ① Chlorides: The corrosion of carbon steel caused by chlorides is essentially uniform corrosion, accompanied by hydrogen embrittlement, while the corrosion of stainless steel is either pitting corrosion or intergranular corrosion. Preventive measures can involve selecting appropriate materials, such as carbon steel-stainless steel composite pipes. ② Sulfides: There are over 250 types of sulfides in crude oil. Those that cause corrosion to metals include hydrogen sulfide (H2S), thiol compounds (R-SH), and thioether compounds (R-S-R). The high H2S content in liquefied petroleum gas in our country causes cracks to appear in the containers; some of them developed through-cracking as early as 87 days after operation. Magnetic particle testing revealed a total of 417 cracks in the circumferential seams on the inner surface, while no cracks were found on the outer surface of the spheres. Therefore, the stress corrosion caused by high H2S levels deserves attention. The Japanese Welding Society and the High Pressure Gas Safety Association stipulate that the H2S content in liquefied petroleum gas should be kept below 100×10-6, whereas the average H2S content in liquefied petroleum gas in China is 2392×10-6, which is more than 20 times higher than that in Japan. ③ Naphthenic acid is an organic compound present in crude oil; corrosion begins to occur when the temperature exceeds 220°C, and it reaches its maximum level at 270–280°C ； When the temperature exceeds 400°C, the naphthenic acids in crude oil have completely vaporized. 316L (00Cr17Ni14Mo2) stainless steel is an effective material against naphthenic acid corrosion, and is commonly used in high-temperature environments subject to naphthenic acid corrosion. 2. Selection of metal materials for pressure pipelines (1) Principles for selecting metal materials ① Meeting the requirements of operating conditions: First, it is necessary to determine whether the pipeline is subject to pressure based on the conditions of use, and what category of pressure pipeline it belongs to. Pressure pipelines of different categories have varying degrees of importance; the severity of the hazards resulting from accidents differs among them, as does the requirement for materials. At the same time, the operating environment of the pipeline, the medium being transported, and the degree of corrosion caused by that medium on the pipe body should be taken into consideration. For example, in the case of steel pipe piles driven into the seabed, the corrosion rate of the pipe body in the wave splash zone is 6 times that in the seabed soil ； The corrosion rate in the tidal range area is 4 times that in seabed soil. Special attention should be paid to material selection and corrosion prevention measures. ② Machinability requirements: The material should have good machinability and weldability. ③ The requirements of durability and cost-effectiveness: Pressure pipelines must first be safe, durable, and cost-effective. For a piece of equipment or a set of piping projects, a feasibility study, that is, an economic and technical analysis, should be conducted when making investment decisions regarding the selection of materials. Several options for the materials to be used can be considered, and an economic and technical analysis carried out on them. Some materials may have a slightly higher initial investment, but they are reliable in use and result in lower maintenance costs over time ； Some materials may seem to require less initial investment, but they have poor reliability during operation, incur high maintenance costs, and result in high total life-cycle costs. (2) Application limitations of common materials The limitations of common materials are shown in Table 1, while their allowable temperature ranges are listed in Table 2. (3) Names and specifications of common metal materials for pipes: The names, standards, grades, and main uses of common steel pipes are shown in Table 3. II. API Standards and Pipeline Steel As early as 1926, the American Petroleum Institute (API) issued the API-5L standard, which initially included only three steel grades: A25, A, and B. Subsequent versions were released over time, as shown in Table 4. Table 4 Pipeline steel grades issued by API. Note: In 1972, API issued the U80 and U100 standards, which were later replaced by X80 and X100. Before 2000, X70 was used worldwide in about 40% of cases, while X65 and X60 were used in around 30% of cases. A considerable number of small-diameter oil pipelines utilized the X52 steel grade, and these were mostly straight pipes made by resistance welding (ERW pipes). Over the past decade or so, China’s metallurgy industry has made tremendous efforts to develop pipeline steel, and it is currently focusing on advancing the development of X70 wide plates. The chemical compositions and mechanical properties of X70 and X80 grades produced by companies such as Shanghai Baoshan Iron and Steel Company and Wuhan Iron and Steel Company are listed in Tables 5 to 9 respectively. Table 5: Properties of Wugang X80 coiled steel plates; Table 6: Mechanical properties of X70-grade steel pipes; Table 7: Results of bending performance tests for X70-grade steel pipes; Table 8: Charpy impact toughness of X70-grade steel pipes; Table 9: Charpy impact toughness of high-strength transmission pipes. The commonly used pipe types in oil transmission pipelines in China currently include spiral submerged arc welded pipes (SSAW), straight-seam submerged arc welded pipes (LSAW), and electric resistance welded pipes (ERW). Seamless steel pipes are used when the diameter is less than 152 mm. In China, from the late 1960s to the 1970s, spiral welded pipe factories developed rapidly; almost all crude oil pipelines used spiral welded pipes, and such pipes were also employed in the first-phase sections of the West-East Gas Pipeline project. The disadvantages of spiral welded steel pipes are high internal stress, poor dimensional accuracy, and a high probability of defects. According to expert analysis, a \"two-pronged approach\" should be adopted: first, actively carry out technological upgrades in existing spiral welded pipe factories, as there is great potential in this area ； Second, vigorously develop China’s straight-seam submerged arc welding pipe manufacturing industry. ERW steel pipes feature a smooth surface, high dimensional accuracy, and low cost, and are widely used both domestically and internationally. III. Steel Tubes 1. Seamless steel tubes for low-pressure and medium-pressure boilers (GB 3087–1999) ① The grades, dimensional specifications, and applications of seamless steel tubes for low-pressure and medium-pressure boilers are shown in Table 10. ② The chemical composition and mechanical properties of the steel pipes are shown in Tables 11 and 12. 3. Welded steel pipes for low-pressure liquid transport (GB/T 3091–2001) (1) Grades, chemical composition, and mechanical properties: The grades and chemical composition (as determined by melting analysis) shall comply with the specifications for Q215A, Q215B, Q235A, Q235B in GB/T 700, as well as those for Q295A, Q295B, Q345A, Q345B in GB/T 1591. The mechanical properties shall meet the requirements specified in Table 17. (2) Manufacturing method: Steel pipes are manufactured using resistance welding or submerged arc welding. (3) Process testing ① Bending test: Resistance-welded steel pipes with a nominal outer diameter D of 60.3 mm or less shall undergo a bending test. The bending radius for galvanized pipes is 8D, while that for ungalvanized pipes is 6D; in both cases, the bending angle is 90°. ② Flattening test: Resistance-welded steel pipes with a nominal outer diameter D greater than 60.3 mm shall undergo a flattening test. ③ Hydraulic test: The hydraulic test pressure values are shown in Table 18. (4) Weld bead height: When the steel pipe wall thickness is not greater than 12.5 mm, the weld bead height shall not be greater than 3.0 mm ； When the wall thickness of the steel pipe is greater than 12.5 mm, the weld bead height shall not exceed 3.5 mm. (5) Pipe length: ERW pipes usually have a length of 4 to 12 meters ； Submerged arc welded (SAW) steel pipes typically have a length of 3 to 12 meters. (6) Curvature: Steel pipes with a nominal outer diameter not exceeding 168.3 mm shall be straight, or in accordance with the curvature criteria specified in the agreement between the supplier and the buyer ； For steel pipes with a nominal outer diameter greater than 168.3 mm, the bending degree shall not exceed 0.2% of the total length of the pipe. (7) Pipe ends: For steel pipes with a wall thickness greater than 4 mm, a bevel of 30° + 5°0° can be applied to the pipe ends, with a root thickness of 1.6 mm ± 0.8 mm; the slope at the pipe ends should be less than or equal to 5 mm. (8) Dimensions and weight ① For steel pipes with a nominal outer diameter not exceeding 168.3 mm, their nominal diameter, nominal outer diameter, nominal wall thickness, and theoretical weight shall comply with the specifications in Table 19. ② For steel pipes with a nominal outer diameter greater than 168.3 mm, their nominal outer diameter, nominal wall thickness, and theoretical mass shall comply with the specifications in Table 20. 4. Seamless steel tubes for boilers (1) Tube specifications: Seamless steel tubes for boilers are available in hot-rolled (extruded, expanded) and cold-drawn (rolled) types, in accordance with the standard GB 5310－1995. ① The outer diameter, wall thickness, and theoretical mass are shown in Tables 21 and 22. ② The allowable tolerances for outer diameter and wall thickness are shown in Table 23. ③ The length of steel pipes is usually 4 to 12 mm. When the wall thickness s ≤ 15 mm, the bending degree shall not exceed 1.5 mm/m ； When 15＜s≤30mm, the bending degree shall not exceed 2.0mm/m ； At s30mm, the bending degree shall not exceed 3.0mm/m. The total bending of the header pipe shall not exceed 12 mm. (2) Grade and chemical composition of the steel pipe The grade and chemical composition of the steel pipe are shown in Table 24. (3) Heat treatment schedule for steel pipes The heat treatment schedule for steel pipes is shown in Table 25. ① When the final rolling temperature of hot-rolled 15MoG, 20MoG, 12CrMoG, 15CrMoG, 12Cr2MoG, and 12Cr1MoVG steel tubes meets the normalizing temperature specified in the table, hot rolling can be used as a substitute for normalizing. Reference address of this article: http://www.weldr.net/simple/skill/special/200612/special_39.htm Pipeline Welding Technology: A Review of the Development of Pipeline Welding Technologies at Home and Abroad. China National Petroleum and Natural Gas Pipeline Bureau Research Institute, Xue Zhenkui, Sui Yongli. 0 Introduction: Most of China’s oil and gas resources are located in the northeast and northwest regions, while the majority of consumption markets are found in densely populated areas such as the southeastern coastal regions and the central and southern parts of the country. This significant separation between production and consumption areas poses a major obstacle to the transportation of oil and gas products, thereby hindering the development and utilization of these resources. Pipeline transportation is the best way to overcome this obstacle. Compared to rail transport, it offers a higher capacity, greater safety, and lower costs for transporting oil and gas products; its construction costs are half those of railways, while its transportation costs are only one-third. Therefore, our country **has included the development strategy of \"strengthening the construction of oil and gas pipelines to establish a pipeline transportation network\" in the 10th Five-Year Plan**. According to plans by relevant authorities, over the next 10 years, China will construct 14 oil and gas pipelines, forming a pipeline network comprising “two vertical routes, two horizontal routes, four hubs, and five gas storage facilities,” with a total length exceeding 10,000 kilometers. This indicates that our country is about to enter a peak period for oil and gas pipeline construction. The key natural gas pipeline projects that are under construction or planned in our country include the West-to-East Gas Transmission Project, which is 4,176 kilometers long and requires an investment of 120 billion yuan. Construction began officially in September this year, with the entire project set to be completed by 2004 ； The Senninglan Gas Pipeline Project, with a total length of 950 kilometers, began construction in May 2000. It is now nearing completion, and natural gas has already been delivered to Xining ； The Zhongxian-Wuhan gas pipeline project, with a total length of 760 kilometers, has made significant progress in its preparatory work; out of the 11 tunnels under construction, 4 have already been completed ； The Shijiazhuang-Zhuozhou gas pipeline project, with a total length of 202 kilometers, began construction in May 2000 and is now nearing completion ； The Shijiazhuang-HanDan gas pipeline project has a total length of approximately 160 kilometers ； Double-track gas transmission project from Jingbian, Shaanxi to Beijing ； Double-line project of the Shaanxi Jingbian-Xi’an gas transmission pipeline ； The Shaanxi-Gansu-Ningxia to Hohhot gas transmission project has a total length of 497 kilometers ； The Hainan Island natural gas pipeline project has a total length of about 270 kilometers ； The Shandong Longkou to Qingdao gas pipeline project has a total length of about 250 kilometers ； The China-Russia gas pipeline project has a total length of 2,000 kilometers within Chinese territory ； For the Guangdong LNG project, the work of attracting investment has been completed; it is scheduled to be completed in 2005. Ongoing and planned oil pipelines include the Lancheng-Chongqing refined oil pipeline project, which spans 1,207 kilometers in length and began construction last May ； The China-Russia oil pipeline project, with a length of about 700 kilometers within China ； The China-Kazakhstan oil pipeline project is 800 kilometers long within Chinese territory. In addition, the 2,000-kilometer long refined oil pipeline from Maoming in Guangdong to Guiyang and Kunming, as well as the crude oil pipelines from Zhenhai to Shanghai and Nanjing, are also set to begin construction soon. In addition to the main pipelines, the construction of a large-scale urban gas transmission network must also be carried out concurrently. Faced with such a huge market and such a rare development opportunity, new challenges have arisen for pipeline construction techniques. Under the same flow rate, it is more economical to build one high-pressure, large-diameter pipeline than several low-pressure, small-diameter pipelines in parallel. For example, a gas pipeline with a transmission pressure of 7.5 MPa and a diameter of 1,400 mm can replace three pipelines with a pressure of 5.5 MPa and a diameter of 1,000 mm; the former allows for a 35% reduction in investment costs and a 19% reduction in steel usage. Therefore, increasing the diameter of pipelines has become a sign of technological progress in pipeline construction. Increasing the conveying pressure within a certain range can boost economic benefits. Taking a gas pipeline with a diameter of 1,020 mm as an example, increasing the operating pressure from 5.5 MPa to 7.5 MPa results in a 41% increase in gas transmission capacity, a 7% reduction in material usage, and a 23% decrease in investment costs. Calculations show that if the operating pressure of the gas pipeline can be increased from 7.5 MPa to 10–12 MPa, the gas transmission capacity will increase by another 33–60%. The trans-Alaska pipeline system in the United States has a pressure of up to 11.8 MPa for gas pipelines and 8.3 MPa for oil pipelines, making them the pipelines with the highest operating pressures currently in use. Both an increase in pipe diameter and an increase in conveying pressure require the pipes to have high strength. In recent years, the strength of pipes has improved significantly while maintaining weldability and impact toughness. Since pipeline installation relies entirely on welding techniques, the quality of welding plays a crucial role in determining the overall quality of the project; welding is therefore a key aspect of pipeline construction. Pipes, welding materials, welding processes, and welding equipment are the key factors affecting welding quality. Our country began building large-diameter long-distance pipelines in the early 1970s. The famous \"83\" pipeline project saw the construction of oil pipelines from the Daqing Oil Field to Tieling, from Tieling to Dalian, and from Tieling to Qinhuangdao, which solved the problem of transporting Daqing crude oil outside the region. The pipeline is designed with a diameter of φ720mm; the steel used is 16MnR, in the form of submerged arc spiral welded pipes, with a wall thickness of 6–11mm. The welding process plan is: manual arc welding method, with upward welding technique ； J506 and J507 electrodes are used for welding; pre-heating at 400°C for 1 hour is carried out before welding, with φ3.2 electrodes used for the root pass, φ4 electrodes for the fill pass, and a top pass as well ； The welding power source is a rotary DC arc welder ； The groove is a 60° V-shaped groove, with single-sided welding at the root to achieve double-sided formation. The pipelines built during the ’83 battle in the Northeast have been in operation for 30 years and are still in use today, proving that the engineering approach adopted back then was correct and that the construction quality was excellent. In the early 1980s, the manual downward welding process was promoted; meanwhile, cellulosic and low-hydrogen downward welding electrodes were developed. Compared with the traditional upward welding process, downward welding offers significant advantages such as faster speed, better quality, and reduced consumption of welding material; therefore, it is widely used in the welding of pipe circumferential seams. Since the early 1990s, self-shielding flux-cored wire semi-automatic manual welding has been promoted; it effectively overcomes the weakness of other welding methods in terms of their resistance to wind conditions in outdoor environments. It also features high welding efficiency, as well as good and stable weld quality, and has now become the primary method for welding pipe circumferential seams. The application of full-position automatic welding for pipelines has been under investigation for many years, and significant progress has been made. The West-East Gas Pipeline project serves as evidence of this, with efficiency and quality levels that are unmatched by other welding methods, indicating that China’s welding technology for oil and gas pipelines has reached a high standard. 2 Steel pipes for pipeline construction 2.1 Development history of pipeline steel In the early days, ordinary carbon steels of the C, Mn, Si types were used for pipeline applications; emphasis was placed on mechanical properties in metallurgy, with no strict requirements regarding chemical composition. Since the 1960s, as the pressure and diameter of oil and gas pipelines increased, low-alloy high-strength steel (HSLA) has been adopted, mainly supplied in hot-rolled and normalized conditions. Chemical composition of this type of steel: C≤0.2%, alloying elements ≤3–5%. With the further development of pipeline steel, by the late 1960s and early 1970s, American oil organizations specified three microalloyed controlled-rolling grades—X56, X60, and X65—in the API 5LX and API 5LS standards. This type of steel goes beyond the conventional concepts of steel; it has a carbon content of 0.1–0.14%, with alloying elements such as Nb, V, and Ti added in amounts of ≤0.2%. The mechanical properties of this steel are significantly improved through controlled rolling processes. By 1973 and 1985, the API standards added X70 and X80 steels respectively, and later X100 pipeline steel was developed, with the carbon content reduced to 0.01–0.04%, and the carbon equivalent correspondingly lowered below 0.35; this marked the emergence of steels with multi-element microalloying and controlled rolling/controlled cooling in the true modern sense. The application of pipeline steel in our country started relatively late; most of the oil and gas pipelines laid in the past were made of Q235 and 16Mn steel. “During the Sixth Five-Year Plan period, China began to develop X60 and X65 pipeline steels in accordance with API standards, and these steels were successfully used in pipeline installation alongside imported steel pipes. In the early 1990s, Baosteel and Wuhan Iron and Steel Corporation developed high-strength and high-toughness X70 pipeline steel, which was successfully applied in the Sining-Lanzhou pipeline project. 2.2 Main mechanical properties of pipeline steel The main mechanical properties of pipeline steel are strength, toughness, and mechanical properties in environmental media. The tensile strength and yield strength of steel are determined by its chemical composition and rolling process. When selecting materials for gas transmission pipelines, steel grades with higher yield strength should be chosen to reduce the amount of steel used. However, a higher yield strength is not necessarily better. Too high a yield strength reduces the toughness of steel. When selecting the steel grade, the ratio of the steel’s yield strength to its tensile strength—the yield-to-tensile ratio—should also be taken into account, in order to ensure the quality of tube forming and welding properties. After repeated stretching and compressing, the mechanical properties of steel change, with its strength decreasing; in severe cases, this decrease can be as much as 15%, a phenomenon known as the Bauschinger effect. This factor must be taken into account when ordering steel plates for pipes. An increase of 40-50 MPa based on the minimum yield strength of steel at that grade can be adopted. The fracture toughness of steel is related to its chemical composition, alloying elements, heat treatment processes, as well as the thickness and orientation of the material. The contents of C, S, and P in steel should be minimized as much as possible. Appropriate amounts of alloying elements such as V, Nb, Ti, and Ni should be added. Processes like controlled rolling and controlled cooling should be employed to enhance the purity of the steel, ensure uniform material properties, and refine its grain structure, thereby improving the steel’s toughness. The common approach currently taken is to reduce C and increase Mn. In oil and gas environments containing hydrogen sulfide, hydrogen generated by corrosion penetrates into pipeline steel, leading to hydrogen-induced cracking. Therefore, pipeline steel for transporting acidic oils and gases should have a low sulfur content, along with effective control over the morphology of non-metallic inclusions and reduction of microstructural compositional segregation. The hardness value of pipeline steel also has a significant impact on HIC; to prevent hydrogen-induced cracks in the steel, it is generally considered that the hardness should be kept below HV265. 2.3 Weldability of pipeline steel: As the carbon equivalent of pipeline steel decreases, its sensitivity to weld hydrogen-induced cracking is reduced; fewer process measures are required to prevent crack formation, and the degree of performance degradation in the weld heat-affected zone is diminished. However, since pipeline steel undergoes a series of complex, non-equilibrium physicochemical processes during welding, defects may arise in the welded area or the performance of the joint may be reduced, primarily due to welding cracks and the embrittlement of the weld heat-affected zone. Due to its low carbon content, pipeline steel has a reduced tendency to harden and a lower susceptibility to cold cracking. However, as the strength level increases and the plate thickness grows, it still exhibits a certain tendency to develop cold cracks. During on-site welding, welding materials with high hydrogen content such as cellulose electrodes and self-shielded flux-cored wires are often used; the low line energy and rapid cooling rate increase the susceptibility to cold cracks, hence it is necessary to take appropriate welding measures such as preheating before welding. Brittleness in the weld heat-affected zone is often the root cause of pipeline fractures, leading to catastrophic accidents. Local embrittlement occurs mainly in two areas: embrittlement of the coarse-grained region in the heat-affected zone, which is caused by the excessive growth of grains in the overheated area as well as the formation of poor microstructures; and recrITICAL embrittlement of the coarse-grained region during multi-pass welding, that is, the coarse-grained region from the previous pass is re-heated by the two-phase region of the subsequent passes. The toughness can be improved by adding a certain amount of Ti and Nb microalloying elements to the steel and controlling the post-weld cooling rate to obtain an appropriate t8/5 value. 2.4 Steel pipes for the West-to-East Gas Transmission pipeline project: The steel pipes used in this project are of X70 grade pipeline steel, with specifications of Φ1,016mm×14.6–26.2mm. Of these, spiral-welded pipes account for about 80%, while straight-seam submerged-arc welded pipes account for about 20%. The total amount of pipeline steel used is approximately 1.7 million tons. In addition to containing Nb, V, and Ti, X70 pipeline steel also includes small amounts of Ni, Cr, Cu, and Mo, which delays the formation of ferrite to lower temperatures, facilitating the formation of acicular ferrite and lower bainite. Therefore, X70 pipeline steel is essentially a high-strength, high-toughness pipeline steel of the acicular ferritic type. The chemical composition and mechanical properties of the steel pipes are shown in Table 1 and Table 2. 3 Welding processes 3.1 Characteristics of field welding Since the discovered and exploited oil and gas fields are located in remote areas with harsh geographical, climatic, and geological conditions, as well as poor social infrastructure, construction faces numerous difficulties; among these, problems caused by low temperatures are the most significant. During field welding, a pipe aligner is used for aligning the pipe ends. To improve efficiency, base beams or mounds of soil are generally placed under the good pipe ends, and preparations for the next jointing begin while welding the previous joint. This will generate relatively large additional stresses. Meanwhile, due to the thermal expansion and contraction of steel pipes, problems caused by additional stress are most likely to occur during butt welding. The on-site welding positions involve horizontal or inclined butt joints of pipes, including flat welding, vertical welding, overhead welding, horizontal welding, and other welding positions. Therefore, higher and stricter requirements are placed on the welding workers’ operational skills. The modern pipeline industry requires pipelines to have high transmission pressures and large diameters, while also ensuring their safe operation. To meet the requirements of higher strength and toughness in pipeline steel, as well as larger pipe diameters and thicker wall thicknesses, various welding methods, welding materials, and welding processes have been developed. 3.2 Pipeline construction welding methods: The development of pipeline welding in foreign countries has gone through the stages of manual welding and automatic welding. Manual welding mainly involves downward welding with cellulose electrodes and downward welding with low-hydrogen electrodes. In terms of automatic pipeline welding, there is the flash butt welding machine for pipelines developed by the former Soviet Union; during the Soviet era, it welded tens of thousands of kilometers of large-diameter pipelines in total. Its notable features are high efficiency and strong adaptability to the environment. The CRC multi-head gas-shielded pipe automatic welding system developed by the American company CRC consists of three main components: a pipe end beveling machine, a combination system of an internal alignment device and an internal welder, and an external welder. To date, the total length of pipes welded worldwide has exceeded 34,000 km. France, the former Soviet Union, and other countries ** have also researched and applied similar technologies for automatic welding both inside and outside pipes. This technological direction has now become the mainstream approach in the field of automatic welding of large-diameter pipes worldwide. China’s ring weld welding technology for steel pipelines has undergone several major transformations. In the 1970s, traditional welding methods were used, namely manual arc welding with low-hydrogen electrodes in an upward welding direction. In the 1980s, manual arc welding in a downward welding direction was adopted, using cellulose electrodes and low-hydrogen electrodes. In the 1990s, self-shielded flux-cored wire semi-automatic welding technology was put into use. Today, full-position automatic welding technology is being widely promoted. Manual arc welding involves the use of cellulose electrodes and low-hydrogen electrodes. The upward welding technique in manual arc welding was the main welding method used in pipeline construction in China in the past. It is characterized by a large gap between the pipe ends during alignment, the use of an arc interruption method during welding, a large thickness for each weld layer, and low welding efficiency. Downward welding using manual arc welding is a welding technique introduced from abroad in the 1980s. It is characterized by a small gap between the pipe ends; during welding, high current levels, multiple passes, and rapid welding methods are employed, making it suitable for assembly line operations and offering high welding efficiency. Due to the relatively thin thickness of each weld layer, the heat treatment effect of subsequent weld layers on preceding ones can improve the toughness of the girth weld joint. The manual arc welding method is flexible and simple, with strong adaptability. The combination of the downward and upward welding techniques, along with the excellent root welding capabilities of cellulose electrodes, means that it remains irreplaceable by automatic welding methods in many situations. The semi-automatic welding technology using self-protecting flux-cored wires has been applied in pipeline construction since the 1990s; it is primarily used for filling and surfacing. It is characterized by high deposition efficiency, good formability in all positions, strong adaptability to different environments, and ease of use for welders; it is therefore an important welding technique for pipeline construction at present. As the strength grades of steel pipes used in pipeline construction increase, along with larger pipe diameters and wall thicknesses, automatic welding technology has gradually begun to be employed in pipeline construction. Due to its advantages such as high welding efficiency, low labor intensity, and minimal impact of human factors on the welding process, automatic pipeline welding technology holds great potential for use in the construction of large-diameter, thick-walled pipelines. However, pipeline automatic welding technology in our country is still in its infancy; the issue of automatic root welding has not been resolved, and supporting equipment such as pipe end groove shaping machines is not yet mature, all of which limit the widespread application of automatic welding technology. At present, ST is primarily used for automatic root welding. Reference address: http://www.weldr.net/simple/skill/html/content_1270.htm Trends in the development of welding technologies for high-pressure and ultra-high-pressure natural gas pipelines I. Introduction With the development of the petroleum, natural gas, and petrochemical industries, and marked by projects such as the West-East Gas Transmission Project, China has entered a period of peak construction activity for long-distance pipelines. Long-distance oil and gas pipelines are increasingly moving toward larger diameters and higher pressure levels for transportation. Since the downward welding technique for long-distance pipelines was introduced to China in the 1960s, decades of development have led to the establishment of mature manual downward welding methods, while semi-automatic gas shielded welding techniques are gradually becoming more widespread. The combination of fully automatic gas shielded welding technology and downward welding technology, as a development trend in high-pressure pipeline welding technology, will be widely promoted in the construction of long-distance pipelines nationwide. Since the manual downward welding technique and the semi-automatic welding process using self-shielded flux-cored wires have been successfully applied in the Shanghai I–VI sections of the \"West Gas to East\" project, and comprehensive construction procedures already exist, they are not repeated herein. As for semi-automatic and fully automatic gas shielded welding using STT technology, no practical applications or welding tests have been carried out on gas pipelines in the Shanghai area. Therefore, this paper provides a detailed discussion of these two welding processes to serve as a technical reference for undertaking future high-pressure gas pipeline projects in Shanghai. II. STT technology – CO2 gas-shielded semi-automatic downward welding technique. The STT type of CO2 semi-automatic welding is used for the root welding of pipes, with CO2 being the shielding gas used in this process. A welding process for manual welding with automatic wire feeding, using flux-cored wires such as Lincoln’s NR207. STT is the abbreviation for “Surface Tension Transfer,” which refers to the effect of surface tension; it is a mechanism involved in the transfer of weld metal. (The filler cap welding using this process is similar to the filler cap welding technique of the semi-automatic welding method employed by our company; no further details will be given here. What is described mainly refers to the STT root welding technique.) ) 1 Process characteristics: In the welding of pressure pipelines, STT welding is a cost-effective and efficient welding method. Traditional CO2 gas shielded welding cannot fundamentally solve the problems of excessive welding spatter and unsatisfactory weld shape. The STT-type CO2 semi-automatic welding machine, which employs waveform control technology, ensures a stable welding process, attractive weld morphology, minimal impact from changes in dry elongation, significantly reduces spatter, and lessens the workload on welders. 2 Process principle: Since the droplet transfer in STT technology relies on the surface tension of the liquid metal, it represents a welding method that makes use of dynamic control in a unique manner. Therefore, the weld pool formed on the weld bead is very small and concentrated; its excellent properties have expanded the application scope of CO2 semi-automatic welding in the construction of long-distance pipelines. 3 Engineering examples: China Petroleum Pipeline Bureau utilized the STT-type semi-automatic downward CO2 welding technology for the root welding of pipelines for the first time in the Muglad oil development project in Sudan. The Zhongyuan Petroleum Exploration Bureau Construction Group Corporation employed this STT-type semi-automatic downward welding technology in the construction of the Beijing-Shijiazhuang section of the Shaanxi-Beijing pipeline duplication project. 4 The welding parameters are shown in Table 1. 5 Welding equipment: As far as the author is aware, at present, there is only Lincoln Company that manufactures STT welding equipment. III. Fully automatic gas-shielded downward welding technology: Fully automatic gas-shielded welding is a welding process that uses fully automatic welding machines and shielding gas for pipeline welding, with argon or CO2 typically being used as the shielding gas. 1 Process characteristics: Since gas shielded welding with a melting electrode provides simple protection for the welding area, it is easy to observe this area; the production efficiency is high. The welding process is relatively simple to control, and full-position welding can be easily achieved. However, it has high requirements for the quality of pipe grooves and alignment, that is, it demands even alignment around the entire circumference of the pipes ； Strict requirements are placed on the type of groove. When the wall thickness of the pipe is large, composite or U-shaped grooves should be used when determining the welding process; it is not sufficient to focus solely on reducing the workload. What is more important is to ensure that the groove contributes to maintaining good welding quality. Although a low-angle V-shaped groove simplifies the welding procedure, from the perspective of ensuring quality, composite or U-shaped grooves are superior ； In addition, it is greatly affected by external climate conditions, which is also a common issue with gas shielded welding ； The gas supply issue is easier to resolve in eastern regions such as the area around Shanghai. 2. Process principle: The fully automatic gas shielded welding technology for pipes uses the arc between the fusible weld wire and the metal to be welded as a heat source to melt the weld wire and the steel pipe. During welding, a shielding gas is supplied to the welding area to prevent the harmful effects of air, and welding is completed through continuous wire feeding. This process enables simultaneous operation of multiple heads in all positions; the root welding can be carried out either from the inside or the outside of the tube, and the adjustment of welding process parameters is generally done via a console or control panel. 3 Engineering example: Zhengzhou-Yima gas pipeline project (east section). The steel pipe material is 16Mn, with a diameter of 426 mm; the welding wire is H08Mn2SiA, with a diameter of 1.0 mm. In the \"West-to-East Gas Transmission\" project, fully automatic gas shielded welding has also been widely used. 4 Welding parameters: It should be noted that since the welding parameters vary for each weld pass, the parameters for the entire weld must be determined based on the specifications of the pipe material and the site conditions; they can only be used in production after passing welding tests. Table 2 shows example parameters for fully automatic welding using flux-cored wire + gas shielded welding. Welding equipment includes the NOREAST fully automatic welder from the UK, the RMS automatic external welder from Canada, and the PIPELINER fully automatic welder. 6 Comprehensive analysis: In semi-automatic gas shielded welding or fully automatic welding, root welding remains a critical process. In the construction of high-pressure gas pipelines, semi-automatic gas shielded welding is used, with STT welding for the root pass and manual welding for the fill and cover passes. The fully automatic welding techniques that can be employed in the construction of high-pressure gas pipelines include: external welding, internal welding, STT technique for root welding, as well as forced shaping of root welding with an external copper gasket followed by external automatic welding. For hot welding in fully automatic welding, processes such as flux-cored wire + gas shielded welding, solid wire + gas shielded welding, and gas tungsten arc welding can be used. Based on the above analysis and the application of various welding methods, under normal circumstances, self-shielded semi-automatic welding should be the primary method for welding long-distance pipelines. For pipeline projects such as the West-East Gas Pipeline, automatic welding is suitable for use in flat areas like Shanghai, while traditional manual welding methods and manual downward welding methods serve as supplementary options. IV. Summary: Pipelines are welded using STT gas-shielded root welding, self-shielded flux-cored wire semi-automatic welding, as well as fully automatic gas-shielded welding techniques. Thanks to their advantages such as high welding quality and fast welding speed, these methods have become widely used abroad, while they are still in the stage of promotion in China. They hold great application prospects in flat areas and under favorable working conditions. Our company should intensify research and testing on these two welding processes, in order to equip itself with the necessary technical capabilities for the construction of high-pressure and ultra-high-pressure pipelines, thus preparing to capitalize on the huge opportunities presented by the development of the natural gas market.

First, it should have a strong sense of three-dimensionality in space; the elements designed need to be arranged reasonably to facilitate construction. Secondly, it is necessary to take into account the operational requirements and the conditions of the actual site; for example, valves should be placed as low as possible on the ground, and the height of pipelines in various passages must also be considered. Otherwise, the designed system will be difficult to operate, with pipes lying everywhere and making it impossible to walk through. Thirdly, they are easy to maintain; for example, production workers dislike flanges because they introduce an additional point of leakage, while equipment technicians prefer that the pipelines be easily removable so as to facilitate equipment maintenance. IV. Be familiar with the process flow diagram and process piping diagram ； V. According to the piping diagram, start with the main pipeline sections first to ensure that there are no conflicts in the overall layout of the pipes, and then proceed with the piping in the detailed areas, taking aesthetics into account as well ； VI. Careful verification is required when taking out valves, pipes, and pipe fittings to avoid incorrect use of materials ； VII. The installation of control valves, flow meters, etc. requires careful adherence to the installation requirements specified for these instruments; otherwise, it will lead to significant errors in operation ； VIII. Control of pipeline welding quality: Many construction companies, in an effort to speed up the progress, are reluctant to pay attention to many details; proper supervision is necessary, especially regarding aspects such as the preparation of the weld sites and gas shielded welding ； 9. Additionally, be sure not to forget things such as pipe expansion joints, pipe U-shaped liquid seals, sampling ports, drain ports, and the slope required for pipe installation ； : 10. After installation, it is necessary to keep proper records of pipeline pressure testing and cleaning/purging. Pressure pipelines require inspection, and relevant personnel from the special inspection agency should be invited to supervise the testing process. 11. For the personnel on site, keeping a construction diary is essential; on one hand, it provides a record for reference, and on the other hand, it helps them to summarize their work and improve their professional skills. 12. I forgot to mention earlier the construction of pipe supports and hangers; there should be ready-made construction manuals available for reference. The type of support or hanger to be used should be determined based on factors such as the medium flowing in the pipe and the operating temperature. Special attention should be paid to those pipes that experience linear expansion